1. Field of the Invention
The present invention relates to a stack structure for fuel cells, and more particularly, to a stack structure in which a plurality of fuel cells are stacked to increase the amount of electric power.
2. Discussion of Related Art
Generally, a fuel cell is a generator which includes a cathode layer and an anode layer on respective surfaces of an electrolyte layer, and generates electricity by an electrochemical reaction between hydrogen and oxygen through ion conduction occurring at the electrolyte layer when an air including oxygen and a fuel gas including hydrogen are supplied to the cathode layer and the anode layer, respectively.
Recently, such a fuel cell is a high efficiency and pollution-free generator having a simple energy conversion process, and generating energy fundamentally through oxidation of hydrogen. Because of such an environment-friendly characteristic, recently, studies of fuel cells are actively progressing.
Particularly, among fuel cells, a solid oxide fuel cell (SOFC) is a fuel cell operated at a high temperature of approximately 600 to 1000° C. using a ceramic as an electrolyte, and has various advantages of the highest efficiency among the various types of fuel cells including a molten carbonate fuel cell (MCFC), a phosphoric acid fuel cell (PAFC), a polymer electrolyte fuel cell (PEFC), etc., less pollution, and enabling combined cycle power generation without a fuel processor.
A fuel cell having a set of an electrolyte layer, a cathode layer and an anode layer is usually called a single cell. Since an electricity generated by the single cell is approximately less than 1 V, which is useless, a technique of increasing a generated voltage by stacking a plurality of single cells in the form of a stack structure has received attention.
Such a stack structure must need a plurality of interconnectors having channels, which are disposed between the single cells to be electrically connected with each other and supply an air or a fuel gas at a location in contact with the cathode layer and the anode layer of the single cell. In addition, each interconnector has a sealing unit formed of a glass material, which is one of ceramic materials sealing the interconnector not to substantially mix the air or fuel gas, and must need an edge region corresponding to a surface extended from an end of the single cell to supply the air or fuel gas.
In this case, when the single cells and the interconnectors are formed to very small thicknesses to reduce a volume of the stack structure, a frame is further included in the edge region to reinforce a strength between the interconnectors and thus prevent bending or sagging caused by heat and weight during operation. In addition, a metal material is included in the frame to reinforce the strength.
In this case, during the operation of the fuel cells at the high temperature, the metal material is volatilized from the frame or diffused at an interface and then transferred to the cathode layer in contact with the air, and the metal volatile matter or diffusate generated thereby reacts with the material included in the cathode layer, thereby generating a composite oxide. Accordingly, due to the composite oxide, an electroconductive property of the single cell is degraded, and thus the electricity generating performance of the single cell may be degraded.
The sealing unit is also disposed between the frame and the interconnectors, and in this case, since the frame including the metal material and the sealing unit formed of the ceramic material are formed of materials having different properties, the frame and the sealing unit may be separated due to the difference in properties of the materials while a temperature at which the fuel cells are operated is dramatically changed, leading to the breakdown of sealability.
In addition, impurities may be generated by a different metal volatile matter generated from a tube for supplying the air or fuel gas or a metal material on a pathway thereof, a reaction phase formed outside the sealing unit by the metal material volatilized from an end of the interconnector or frame, an oxide scale formed in a part in which the interconnector faces the frame outside the sealing unit, or an insulating material fragment or dust that may be present in an external space of the sealing unit, and in this case, the impurities may produce a reaction phase along a circumference of the sealing unit, interconnector or frame, leading to an electrical shunt phenomenon in which an electric current overall flows.